Pili, assembled from pilin subunits, are fibers on the surface of bacteria that allow attachment to cell surfaces. John Tainer's group (at Scripps) have used x-ray crystallography to determine the atomic structure of pilin from Neisseria gonorrhea. Pilin has a modular design with a striking 85 `-helical spine that merges with a globular `- roll. In collaboration with his group, we are using cryo-EM and helical image processing to derive a molecular envelope for the gonoccocal pilus in which the atomic model can be docked. Mass/length measurements by the STEM are being used to evaluate different packing models for the pilin subunits within the fibers.